Can I edit an active sequence?

Absolutely..! You can modify almost everything in a sequence even after it starts. 

What you can change in an active sequence: 

Schedule: You can change the schedule for the sequence from the Settings tab inside the sequence. You can also modify the used schedule from Settings > Schedules. 

Content: You can modify the email content of any step. You can change the subject line. You will be able to add a new variant in an existing step or add a new step in the sequence. 

Prospects: You will be able to add new prospects either manually or using a CSV file. You can also remove any existing prospect from the sequence. 

What you can't change in an active sequence: 

Email Account: Once a sequence is activated, it's not possible to change the email account that was chosen for that sequence. If you really need to use a different email account, we'd suggest you duplicate the sequence to create a clone of that and then activate it using a new email account. 

Deletion of step in which email is already sent: You won't be able to delete a step after an email is already sent in that step or you have added a prospect to that step.